The crystallization of Metal Organic Frameworks (MOFs) on the surface of organic and inorganic membrane materials has picked up recent scientific attention. It has been shown that MOF membranes and thin films synthesized in this way have potential application in separation of industrial gases and liquids, amongst several more potential applications, including solar cells, fuel cells, drug delivery, healthcare, optoelectronics and so on and so forth. MOF based membranes have been synthesized using inorganic porous supports like alumina, zeolites, silica, graphite, carbon, etc. Though these inorganic materials possess high thermal stability, their usage is limited because of their brittleness and lack of flexibility, hindering their fabrication into continuous sheets for practical applications. In order to overcome these limitations, recently researchers have attempted usage of flexible organic polymeric membranes like polymethylmethaacrylate (PMMA), polyimides, matrimids, polysulfone and similar materials to fabricate MOF polymer composites. These flexible polymeric membranes can effectively be fabricated into flexible sheets, commercially beneficial patterns, and are amenable to easy large scale production and possess high pressure stability.
Harold B et al, cited in Dalton Trans., 2012, 41, 14003 report Mixed Matrix Membranes (MMM's) with metal-organic frameworks such as CuBTC, Zeolites as additives exhibiting gas permeability and selectivity. Polyimides and polysulfone were the general polymer matrices for MOF fillers used herein. The researchers have suggested that pore blockage around the interface region of the MOF may possibly lower the permeability; however the study is silent on a method for fabrication of MOF-polymer composites.
Common methods used to synthesize MOF-polymer composites include blending the pre-synthesized MOFs with the polymer matrix via secondary seeded growth processes or by functionalizing the substrate using —COOH or pyrimidine surface terminated substrates.
Liu et al in Materials 2012, 5, 1581-1592 describe deposition of metal-organic frameworks (MOF) by liquid-phase epitaxy (LPE) on —COOH terminated templating organic surfaces prepared by the adsorption of self-assembled monolayers (SAM's) on gold substrates to form MOF thin films. The influence of the packing density of the carboxylic acid group on the orientation and quality of MOF thin films grown using the LPE is determined.
The epitaxial growth process consists of alternately immersing an appropriate SAM substrate into ethanolic solutions of copper acetate and H3BTC (benzene-1,3,5-tricarboxylic acid). The solutions were kept at 50° C. during MOF thin film preparations.
Lui et al teaches the process of alternately immersing SAM into a ligand and the metal solution; however the number of cycles for subjecting SAM's is also, not specified for immersing the same.
Lei Ge et al in The Journal of Phys. Chem. 2012, 116 13264-70, describes a hydrothermal route for synthesis of ZIF-8 thin layer on the asymmetric porous polyethersulfone (PES) substrate via secondary seeded growth. In such processes the thickness and microstructure of the polymeric membranes should be optimized by the synthesis conditions to improve gas separation performance.
Further, fabrication of free-standing MOF membranes using nanofibrous mats produced by electrospinning as a new type of porous support is reported by Qui et al in Chem. Eur. J. 2012, 18, 10250-53, a two-step procedure was developed, namely the preparation of MOF nanocrystal doped electrospun fibers followed by a second growth.
MOF-polymer composites achieved by blending pre-synthesized MOF's with polymer membranes often results in sedimentation and agglomeration of crystals. High loading of MOF crystals in the polymer matrix results into brittleness. Moreover, by blending, opening of nanocrystals may be covered by polymer chains, thus hindering direct contact of the penetrant with nanocrystals. However, in the instant invention MOF's are formulated, and then applied on porous polymer to form the MOF-polymer composite.
PCT Application: WO/2012/11212 is directed to a mixed-matrix composite (mixed-matrix membranes (MMMs) comprising a continuous phase and zeolitic imidazolate framework (ZIF) particles dispersed in the continuous phase, wherein the continuous phase is polybenzimidazole (PBI)).
The invention claims a process of forming a mixed-matrix composite material, comprising mixing ZIF particles into PBI solution for a sufficient amount of time to allow ZIF particles to uniformly disperse in the PBI solution; and fabricating the solution to thereby produce mixed-matrix composite material comprising of PBI and ZIF particles dispersed in the continuous phase.
The drawback of this process is that the ZIF particles may be randomly positioned on the polymer membrane composite thus affecting the gas separation efficiency of the composite system.